Magnetic recording and reproduction



Patented Sept. 19, 1950 UNITED STATES PATENT OFFICE MAGNETIC RECORDING AND REPRODUCTION No Drawing. Application July 22, 1948, Serial No. 40,164

11 Claims. 1

This invention relates to magnetic recording and reproduction and, more particularly, to sound wire therefor.

The requisite magnetic and mechanical properties of sound Wire for magnetic recording and reproduction are disparate from those which are important in massive magnets. In fact, many of the desired qualities of a sound wire are such that they cannot adequately be evaluated or predicted on the basis of measurable magnetic properties of the wire but must be ascertained by actual use of the wire in a magnetic recording and reproducing machine.

The important magnetic characteristic of massive magnets is that they have the maximum value for the product of their coercive force (He) and residual magnetism (Br) The ultimate mechanical properties of such massive magnet materials are that they have sufficient strength to resist mechanical shock and that they be sufficiently machinable to permit fabrication to precise dimensions.

The important magnetic characteristics of sound wire are manifold. These characteristics apply to sound wire in the broad sense in which this term is used herein and in the claims. As so used, sound wire includes both small diameter wire and thin ribbon of the type now used in magnetic recording machines for the recording of sound and for the recording of pulses or the like in computers and various memory devices. It is desirable, for example, that sound wire have a high value for the product of its coercive force and residual magnetism. The usefulness of this magnetic power is lessened, however, by the level of the background noise of the wire. This usefulness, which is known as dynamic range, is therefore determined by the magnetic power of the wire minus its background noise. The magnetic power is influenced by the composition, heat treatment, and working state of the wire. The background noise is aggravated not only by surface impurities such as oxides but also by the presence within the composition of elements forming refractory oxides.

Although the product of the coercive force and residual magnetism is important in determining, the dynamic range of a sound wire, the relationship between the coercive force and the residual magnetism is also of importance. The response of the wire to frequencies above 2000 cycles per second is strongly influenced by the ratio of the residual magnetism to the coercive force; the smaller this ratio, for a given value of the residual magnetism, the better the response to high frequencies.

The utility of sound wire is also determined by its magnetic qualities known as transfer and erase. Transfer is the term given to the tendency for magnetization of one portion of a Wire to magnetize an adjoining portion of the wire in an adjacent turn of the coil or spool of wire. The transfer characteristic of sound wire appears to be related to the initial slope of the magnetization curve of the wire, the lower this initial slope the lower the transfer. Where it is desired that the wire be capable of repeated usage for the recording thereon of different signals, the wire must be capable of being electrically treated so as to remove or erase therefrom any previously recorded signal and place the wire again in condition for the recording of a new signal thereon. The erase characteristic of sound wire appears to be related to the high frequency response and magnetic saturation of the wire.

The mechanical properties of sound wire are also subject to certain demanding qualifications which are not required for massive magnets. Sound wire must have sufficient workability to permit it being drawn or rolled into the desired finished form such as small diameter wire or thin ribbon. The finished product, referred to generically herein as wire, and having the desired magnetic characteristics, must also have sufficient tensile strength to withstand the starting and stopping tensions imposed upon it in a recording machine. The finished product should also be ductile. In the form of small diameter wire, the ductility should be such that the Wire is capable of being tied in a knot which will set without the ends of the knot slipping and pulling apart. Such knots are the standard expedient used for joining sections of recorded passages to produce a continuous record.

Ferromagnetic alloys suitable for the production of massive magnets have been proposed heretofore comprising copper, nickel, iron and cobalt. For example, the United States Patent to Buchner et al., No. 2,124,607, describes such alloy compositions containing 10 to nickel, 20 to 75% desirable for use in the production of massive magnets. Both patents describe a variety of combinations of heat treatments which are applicable to the production of magnets of massive form.

We have now found that sound wire for magnetic recording and reproduction which is characterized by unusually high sensitivity, by low transfer with low background noise and satisfactory erasing qualities, and by the adequate tensile strength and ductility, may be produced from alloys composed of 15 to 27% nickel, 12 to 25% of at least one metal of the group consisting of cobalt and iron, and the balance copper except for the inclusion of small amounts of other metals as hereinafter discussed. The production of sound wire from such an alloy composition is predicated upon a critical heat treatment and cold working schedule. Thus, in order to obtain sound wire from an alloy of composition within the aforementioned ranges, the wire must be heated in the cold worked state to a temperature of 8'75-1175 C. while at a size having a crosssectional area from 15 to 70 times as large as that of the finished wire size, then cold worked to a smaller size. subsequently reheated to a temperature of 675-725 C. while at a size having a cross-sectional area from 2 to 8 times as large as that of the finished wire size, and finally cold working to the finished size.

The sound wire of our invention excels in sensitivity over all other sound wires and tapes which are presently available. This sensitivity includes not only an exceptionally large dynamic range, by virtue of high coercive force and high residual magnetism together with a low background noise level. but also excellent res onse to a wide range of frequencies. The sensitivity of the sound wire of the invention, when expressed quantitatively in terms of the energy product of the coercive force and residual magnetism, is approximately five times as great as that of the best stainless steel sound wire. The transfer characteristic of the sound wire of our invention does not exceed its low background noise 1evel and in many instances is actually zero. The erase characteristic varies from one extreme to the other, depending upon the com osition of the wire and the heat treatment which it receives. Thus, the composition and heat treatment may be so chosen that the wire is readily erased, or a composition and treatment may be selected which will insure exceptional permanence for the recording. The sound wire of our invention is also characterized by sufiicient tensile strength and by a ductility such that ends of small diameter wire may be permanently joined by tying them in a knot.

The nickel content of the sound wires of our invention must be kept within the range of 15 to 27%, and preferably within the range of 20 to 25%. We have not produced products of satisfactory magnetic recording qualities with wire containing less than 15% nickel. Sound wires of most effective combinations of high fidelity and sensitivity, low transfer and low background noise are generally those containing from 20 to 25% nickel. As the nickel content exceeds 25%, the noise level begins to rise with the result that the production of satisfactory sound wire requires limiting the nickel content of the wire to a maximum of 27%. The nickel content of our sound wire, to the extent that it exceeds 20%, progressively improves the transfer and erase characteristics of the wire. This progressive improvement per unit increase of nickel above 20 appears to reach a maximum at about 25% nickel. Moreover, amounts of nickel in excess of 20% tend to increase the strength of the wire until, at about 27%, the relatively large amount of nickel causes the strength to fall off.

As indicated hereinabove, the composition of the alloys which we have found amenable to the production of sound wire in accordance with the invention may be varied to the extent that it may contain cobalt without iron or iron without cobalt in addition to the nickel and copper. For example, alloys containing cobalt within the specified range and substantially free from iron appear to have the greatest sensitivity and fidelity as well as the greatest tensile strength. Thus, sound wire having a nominal composition of 20% nickel, 20% cobalt and the balance copper has been found to have exceptionally high fidelity, 10w transfer and high strength. On the other hand, sound wire of excellent quality but somewhat lower cost can be produced from an alloy in which the cobalt is completely replaced by iron. For' example, we have produced sound wire having a nominal composition of 20% nickel, 15% iron and the balance copper. This wire has very high fidelity characteristics with low transfer and good erase. Similar properties were found in sound wire comprising 20% nickel, 20% iron and the balance copper. The strength of this type of wire is lower than that of the iron-free wire and also lower than that of stainless steel wire. On the other hand, the sound wires of our invention are softer than stainless steelwires and are not so wild and so prone to kink or knot as stainless steel w1re.

We have also found is advantageous to include both cobalt and iron in the alloys from which sound wire is produced in accordance with the invention. As previously indicated, the total amount of cobalt and iron should be kept within the boundaries effective for either metal alone. For example, we have produced excellent sound wire having a nominal composition of 20% nickel, 5% cobalt, 15% iron and the balance essentially all copper. The transfer of this wire was low but could be lowered still further, when given the same optimum heat treatment, by increasing the nickel content at the expense of the iron, that is by changing the nominal composition to 25% nickel, 5% cobalt, 10% iron and the balance copper. Sound wire of excellent quality can also be produced by reversing the relative amounts of cobalt and iron. Thus, sound wire having a nominal composition of 20% nickel, 15% cobalt, 5% iron and the balance essentially all copper can be produced having high sensitivity and nearly zero transfer.

The amount of copper predominates in the sound wire of the invention. Th copper content may be as low as 50% and as high as 65%. Below 50% copper the magnetic qualities which characterize the sound wires of our invention rapidly fall off. Amounts of copper in excess of 65% call for such a reduction in the remaining amount of nickel and either iron or cobalt, or both, that the synergistic effects of these metals are largely lost. The copper, as is the case with the other components of the composition, may advantageously be metal of high purity although generally satisfactory results have been obtained with alloys produced from metals of commercial grades of purity.

We have found it useful to add certain other elements to the composition of the sound wires of .1? invention to effect deoxidation, grain rewire.

anaees finement, degassing, and the like. For example, deoxidation may be insured by the addition of up to 0.05% calcium or lithium, or both, up to 0.05% carbon, up to 0.05% phosphorus, or up to 1% silicon or manganese. Increasing amounts of manganese up to 5% may be added without material alteration of the magnetic qualities of the Amounts of manganese in excess of 5% and up to have been found to improve somewhat the workability of the alloys although such an amount of manganese tends to increase the background noise level and lower the coercive force of the wire. A lowered coercive force represents an increase in the ease of erasing the wire although it also represents a corresponding decrease in the sensitivity of the wire. Inasmuch as we have found, as hereinafter more fully explained, that the ease of erase may be improved by control of the heat treatment, we prefer generally to use this latter means for controlling the erase rather than using manganese to effect this result. Moreover, hydrogen or argon or a vacuum maybe used as a non-oxidizing atmosphere in the melting furnace in preparing the alloy compositions. Such atmospheres are not essential, however, inasmuch as satisfactory melting conditions are obtainable with an induction type furnace especially when one of the aforementioned deoxidizers is added to the melt. In addition to the foregoing deoxidizing and degassing elements, up to 1% of columbium, tantalum, titanium, zirconium, vanadium or tungsten, or mixtures thereof, may be incorporated in the alloy composition as grain refining elements.

The foregoing discussion of the characteristics of the sound wire of our invention, and of the effect on these characteristics of varying amounts of the separate components, is predicated upon the sound wire having been heat treated and worked according to a well-defined schedule. Regardless of the composition of the wire, the magnetic sound recording and reproduction qualities of the wire are not present unless the wire has been treated according to this schedule. The treating schedule required for producing sound wire from the aforementioned alloy compositions comprises heating the wire in the cold-worked state to a temperature of 875-1175 C. while the wire is at a certain critical size range with respect to its finished size, then cold working the resulting wire to a smaller size, subsequently reheating the resulting wire at a temperature of WIS-725 C. at this smaller size which also is a certain critical size range with respect to the finished size, and finally cold working the wire to its final or finished sound wire size.

The combination of the two specific heat treatments is a requisite condition for the production of sound wire from the alloys of the described composition. The first of these heat treatments (the solution anneal) effects the first control of the sensitivity of the wire; without this control the second heat treatment (the precipitation) has no beneficial effect on the sensitivity of the wire. We have found that the solution annealing treatment imparts to the wire for the first time the magnetic characteristics which make it suitable as sound wire. However, the annealing treatment is eifective only when applied to wire which previously has been cold worked to a considerable extent. We have also found that the effectiveness of the precipitation treatment in improving the sensitivity and other desirable characteristics of the sound wire depends upon additional cold working of the wire intermediate the solution annealing and precipitation heat treatments. Moreover, the ultimate high sensitivity of the wire is obtained only when the final cold working to finished size follows the precipitation heat treatment.

The details of the heating and working treatment in accordance with our invention will be described hereinafter with respect to the production of fine diameter wire. It will be understood that this treatment is representative also of the production of thin ribbon for sound or impulse magnetic recording.

The drawing operation is preferably carried out with intervening softenings at as low a temperature and for as short a period as is consistent with maintenance of workabilit In general, such softening for workability should be effected at a temperature of substantially 1000 C. and for a period of 15 to 30 seconds. I

The first heat treatment (the solution anneal) which provides control of the magnetic characteristics of the wire is eifected when the wire has been drawn to a size having a crosssectional area ranging between 15 and 70 times (and preferably about times) as large as that of the finished wire size. The cold Worked wire at this size is heated to the annealing temperature preferably in a non-oxidizing or inert atmosphere and is quenched from the annealing temperature. For lowest background noise, the annealing should be effected in a bright annealing atmosphere of nitrogen or hydrogen, or mixtures thereof, free from contained oxygen, moisture and carbon dioxide.

The solution annealing temperature which we have found to be effective ranges between 875 C. and the point where the wire loses its cohesion prior to melting. The maximum upper temperature corresponding to this point is about 1175 C., although for some alloy compositions a somewhat lower temperature of 1050 C. is the maximum. The solution anneal period should be at least 15 seconds with the wire at the anneal temperature and may be as long as more than one hour at the anneal temperature. We prefer the shorter annealing periods for practical reasons, namely, in order to prevent excessive grain growth and to minimize the amount of furnace space required to hold the necessary quantity of wire while it is being run through the furnace for the desired annealing period. The solution annealing treatment is completed in each instance by quenching the wire in a suitable non-oxidizing medium such as a stream of hydrogen or a stream or body of water.

The solution anneal treatment may be the same or nearly the same as a previous softening anneal given to the wire in the course of drawing it down to the requisite size preceding the solution anneal treatment. None of these previous softening treatments, even though they be identical with the solution anneal treatment, has any noticeable effect or influence on the magnetic qualities of the wire. We have found it essential for the attainment of the desired magnetic qualities in the wire that it should be subjected to the abovedescribed solution annealing conditions at a stage in its reduction to finished size corresponding to a cross-sectional area ranging between 15 and times that of the finished wire size. Where the finished wire size is 0.004 inch diameter, the solution anneal treatment should be effected when the wire is reduced to 0.0160.050 inch diameter, and

7 preferably at a diameter of about 0.028 inch. However, as pointed out hereinabove, this high temperature anneal which effects initial control over the sensitivity of the wire is of no value unless it is followed by the precipitation heat treatment.

The precipitation heat treatment must be carried out within a very narrow temperature range and on wire which has been cold drawn intermediate this heat treatment and the prior solution annealing treatment. The temperature of the precipitation reatment should be between 675 and 725 C., best results being obtained by heating at 700 C. The wire should be maintained at the precipitation temperature for at least minutes and not more than 40 minutes. Precipitation does not take place to an effective extent in less than 5 minutes at temperature; and with heating periods in excess of 40 minutes, particularly at the upper range of the precipitation temperature, there is a pronounced tendency for the precipitate to grow in grain size or go back into solution. We have found that in general the optimum magnetic properties of the sound wire are obtained throughout the entire precipitation heating range by a heating period of about 25 minutes. The precipitation heat treatment actually softens the wire and is not a precipitation hardening treatment. The combination of heating temperature and the holding time at this temperature should be suflicient to effect the desired precipitation within the alloy composition without effecting appreciable re-solution of the precipitate. The production of the precipitate during the course of this treatment effects an outstanding increase in the coercive force of the wire, the coercive force of the wire in its solution annealed condition being very low. Heating to an excessive temperature or for an exces-. sive period of time during this precipitation heating treatment lowers the coercive force of the wire. Temperatures appreciably below 675 C. are not effective in producing good sound Wire because at these lower temperatures the transfer characteristics of the wire are bad. Thus, at a precipitation heating temperature of 600 C., it is not possible even with a prolonged heating period to produce a sound wire in accordance with the invention characterized by high coercive force and low transfer. The high transfer characteristics of wire heated at too low a precipitation heating temperature is progressively improved as the precipitation heating temperature is increased. For any given composition within the ranges set forth hereinabove, we have found that the transfer reaches a minimum as the precipitation heating temperature is increased to 700 C. for a period of 25 minutes. Thus, when carried out at a temperature of 675-725 C. and for a period of time ranging between 5 and 40 minutes, the precipitation heat treatment improves the coercive force and energy product (BrXHc) improves the high fidelity characteristic, and improves the magnetic transfer characteristic of the wire.

The effectiveness of the precipitation heating treatment is realized, however, only when this heat treatment is carried out on the wire which has a size corresponding to a cross-sectional area ranging between 2 to 8 times (and preferably 4 times) that of the finished Wire size. Thus, if the finished wire size is 0.004 inch diameter, the precipitation heating treatment should be effected at a wire diameter of 0.008 inch for optimum results. By effecting the precipitation heating treatment at this stage in the production of the sound wire in accordance with our invention, there is assured both the requisite amount of cold working intermediate the solution annealing treatment and the precipitation'heating treatment as well as the requisite amount of cold workin which should follow and climax the precipitation heating treatment. It should also be e noted that in order to maintain a bright surface finish on the wire and thus keep the background noise to a minimum, the precipitation heating treatment atmosphere should be non-oxidizing, and should preferably be a bright annealing atmosphere such as that described in connection with the solution annealing treatment.

The following wire forming and heating schedule is illustrative of the treatment described hereinbefore. The wire is cold drawn down to 0.028 inch diameter with occasional softening treatments at temperatures around 1000 C. for periods of about 15 seconds each in order to facilitate the working to this stage. At this wire size (0.028 inch diameter) the wire is given a solution anneal treatment at approximately 900 C. for a period of 15 seconds by passing it through a strand furnace. The wire is then further drawn cold down to a diameter of 0.008 inch and is then given the precipitation heating treatment by heating it at 700 C. for a period of 25 minutes in another strand furnace. The so-treated wire is then cold drawn directly to its finished size of 0.004 inch diameter. It must be understood, of course, that this schedule is merely illustrative of an effective treating schedule for the production of sound wire having a finished size of 0.004 inch diameter. Thus, where the solution annealing treatment is effected in such a schedule at a wire size above 0.016 inch diameter, two precipitation heating treatments can be effected, one at the aforementioned critical size of 0.008 inch and the other at a larger intermediate size such as 0.016 inch diameter. This schedule may be modified with respect to temperatures and periods of heating as well as with respect to wire sizes for these two treatments, depending upon the desired finished size of the wire and the magnetic characteristics which it is desired to emphasize.

As previously noted, the magnetic qualities of the wire may be controlled to some extent by a choice of heat treating conditions. For example, in many instances the sensitivity of the sound wire increases with an increasing solution-annealing temperature within the specified range of 875-1175 C. Thus, for any given composition, it is possible to obtain either the maximum sensitivity, or the maximum erase with a slightly lower sensitivity, by choosing a solution anneal temperature within the specified range. The specific solution annealing temperature which will give this result can readily be ascertained for any specific composition within the compositional ranges set forth hereinbefore. The transfer characteristic of the finished wire is controlled principally by the precipitation heating treatment as pointed out hereinbefore. For example, when a wire having a nominal composition of 20% nickel, 15% cobalt, 5% iron, 1% manganese and 59% copper is treated in accordance with the invention by solution annealing at 900 C. at a wire size of 0.028 inch diameter for 15 seconds at temperature, is then quenched and reduced to a wire size of 0.008 inch diameter at which size it is heated to 600 C. for 25 minutes and is then reduced to 0.004 inch diameter, the wire has a high sensitivity but an objectionably high magnetic transfer which renders it virtually useless. When the same wire is given the same treatment except that the precipitation heating treatment is raised from 600 to 700 C., the high sensitivity is maintained but the transfer is reduced nearly to zero.

With the transfer thus reduced to the minimum by means of the proper selection of heat treating conditions, the transfer can be further reduced in many instances by changing the composition of the wire. For example, a wire comprising 20% nickel, cobalt, 15% iron, 1%manganese and 59% copper is given high sensitivity with lowest transfer by a treatment in which solution annealing is effected at 1050 C. for 15 seconds at temperature at a wire size of 0.028 inch diameter and precipitation heating is effected at 700 C. for 25 minutes at a wire size of 0.008 inch diameter. When the composition of such a wire was altered by increasing the nickel about 5% at the expense of the iron (that is increasing the nickel to 25% and lowering the iron to while maintaining the other components at their formal value), this same optimum heating treatment produced a wire of high sensitivity and a substantially lower magnetic transfer.

It will be seen, accordingly, that the sound wire of our invention results from a blending of specific elements within critical proportional ranges and a specific schedule of heat treating and working. The combination of these controls over the composition and metallographical properties of the wire makes possible the production of sound wire having the necessary physical properties of strength and ductility and superior magnetic recording and reproduction properties such as high sensitivity, large dynamic range, low transfer and good erase.

We claim:

1. A sound wire for magnetic recording and reproduction characterized by high sensitivity, low transfer and low background noises, the wire being composed of to 27% nickel, 12 to of at least one metal of the group consisting of cobalt and iron, and the balance essentially all copper, the copper content ranging between and 65%, said wire having been solution annealed in the cold worked state at a temperature of 875-1175 C. while at a size having a crosssectional area from 10 to 70 times as large as that of the finished wire size, quenched, then reduced to a. smaller size having a cross-sectional area from 2 to 8 times as large as that of the finished wire size, reheated to a temperature of 675-725 C. for a period of 5 to 40 minutes, and finally reduced to the finished size of about 0.004 inch diameter.

2. A sound wire according to claim 1 in which the nickel content is at least 20% and not more than 25%.

3. A sound wire according to claim 1 having a nominal composition of 20% nickel, 20% cobalt, and the balance essentially all copper.

4. A sound wire according to claim 1 having a nominal composition of 20% nickel, 15% iron, and the balance essentially all copper.

5. A sound wire according to claim 1 having a nominal composition of 20% nickel, 5% cobalt, 15% iron, and the balance essentially allcopper.

6. A sound wire according to claim 1 having a nominal composition of 25% nickel, 5% cobalt, 10% iron, and the balance essentially all copper.

'7. A sound wire according to claim 1 having a nominal composition of 20% nickel, 15% cobalt, 5% iron, and the balance essentially all copper.

8. A sound wire according to claim 1, said wire having been cold drawn to a size having a cross-sectional area about 50 times as large as that of the finished wire size, then annealed at 8751l75 C. for about 15 seconds, quenched, then drawn to a size having a cross-sectional area about 4 times as large as that of the finished wire size, reheated at this latter size to a temperature of about 700 C. for a period of about 25 minutes, and finally drawn to the finished size of about 0.004 inch diameter.

9. The method of producing sound wire by imparting magnetic recording and reproducin qualities to a wire composed of 15 to 27% nickel, 12 to 25% of at least one metal of the group consisting of cobalt and iron, and the balance essentially all copper within the range of 50 to 65% copper which comprises cold working the wire to a size having a cross-sectional area from 10 to '70 times as large as that of the finished wire size, solution annealing the cold worked wire at this size by heating it to a temperature of 8751175 C. for at least 15 seconds, quenching the heated wire, working the wire to a size having a cross-sectional area 2 to 8 times as large as that of the finished wire size, heatin the wire at this latter size to a temperature of 675725 C. for a period of 5 to 40 minutes, and finally working the resulting wire to finished size of about 0.004 inch diameter.

10. The method according to claim 9 in which the wire is solution annealed at a size having a cross-sectional area about 50 times as large as that of the finished wire size and is heated to the range of 675725 C. while at a size having a cross-sectional area about 4 times as large as that of the finished wire size.

11. The method according to claim 9 in which the wire is cold drawn to about 0.028 inch diameter, then annealed at this size at 900 C. for about 15 seconds, quenched, then drawn to about 0.008 inch diameter, heated at this latter size to a temperature of 700 C. for a period of 25 minutes, and finally drawn to a finished size of 0.004 inch diameter.

HUGH S. COOPER. WAYNE E. MARTIN. SOL L. REICHES. JOSEPH B. GRAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,992,325 Schaarwachter Feb. 26, 1935 2,124,607 Buchner et al. July 26, 1938 2,167,188 Schaarwachter et a1. July 25, 1939 2,170,047 Dannohl et al. Aug. 22, 1939 2,257,708 Stott Sept. 30, 1941 2,437,563 Seaver Mar. 9, 1948 

9. THE METHOD OF PRODUCING SOUND WIRE BY IMPARTING MAGNETIC RECORDING AND REPRODUCING QUALITIES TO A WARE COMPOSED OF 15 TO 27% NICKEL, 12 TO 25% OF AT LEAST ONE METAL OF THE GROUP CONSISTING OF COBALT AND IRON, AND THE BALANCE ESSENTIALLY ALL COPPER WITHIN THE RANGE OF 50 TO 65% COPPER WHICH COMPRISES COLD WORKING THE WIRE TO A SIZE HAVING A CROSS-SECTIONAL AREA FROM 10 TO 70 TIMES AS LARGE AS THAT OF THE FINISHED WIRE SIZE, SOLUTION ANNEALING THE COLD WORKED WIRE AT THIS SIZE BY HEATING IT TO A TEMPERATURE OF 875*-1175*C. FOR AT LEAST 15 SECONDS, QUENCHING THE HEATED WIRE, WORKING THE WIRE TO A SIZE HAVING A CROSS-SECTIONAL AREA 2 TO 8 TIMES AS LARGE AS THAT OF THE FINISHED WIRE SIZE, HEATING THE WIRE AT THIS LATTER SIZE TO A TEMPERATURE OF 675*-725* C. FOR A PERIOD OF 5 TO 40 MINUTES, AND FINALLY WORKING THE RESULTING WIRE TO FINISHED SIZE OF ABOUT 0.0004 INCH DIAMETER. 